Producing sand with fluids from an unconsolidated subterranean oil or gas producing zone has long been a problem in the petroleum industry, causing erosion of production equipment and plugging of the well. Such conditions often result in reduced production levels or loss of well production entirely.
An effective means of combating the problem is the gravel pack, which involves placing a tubular liner in the well bore and packing gravel between the tubular liner and the well bore. The liner has slots or other apertures in its walls which are smaller in size than the gravel particles, thereby permitting formation fluids to flow through the slots while preventing entry of the gravel particles. A screen may be provided around the liner to cover the liner slots and thereby provide further assurance against the slots becoming plugged with gravel particles. As a result of improved gravel pack technology, including modern methods of predicting gravel size requirements, gravel packs have become quite effective in excluding sand from oil and gas production. In addition to this function, the gravel also assists in supporting the walls of uncased wells and preventing caving of loose material against the liner. Despite the effectiveness of gravel packs once they are properly placed and operating, the problem of installing a uniform gravel pack has not been satisfactorily solved.
Gravel is commonly mixed with a fluid, for example a liquid or a foam, to form a slurry. Typically, the slurry is transported in the well through the work string to a crossover tool which directs the slurry to the annulus between the liner and the well bore. The slurry continues down the annulus to the bottom of the well bore or to a sump packer in the well bore. Some of the fluid of the slurry flows through the apertures in the liner, enters the open bottom end of a wash pipe situated within the liner, and is returned to the surface through the crossover tool and work string. The balance of the slurry fluid flows into the subterranean zone penetrated by and in fluid communication with the well bore thereby depositing gravel against the subterranean zone.
The problem of depositing a uniform annular column of gravel can be attributed in part to the liner. The slurry fluid flows through the openings in the liner above the top of the deposited gravel, thus reducing the flow of slurry fluid down the annulus between the liner and the well bore. Slurry fluid flow through the liner results in gravel being deposited and subsequently bridging across portions of the annulus which prevents the flow of the slurry to the annulus below the bridge. Such bridging usually occurs near the top of the liner. Efforts have been made to correct this problem by various methods.
One proposal involves the use of large diameter wash pipes so that the annulus between the wash pipe and the liner is small, thus increasing the resistance to flow through this smaller annulus and causing the slurry fluid to preferentially flow down the annulus between the well bore and the liner rather than pass through the liner openings. This arrangement has not resulted in a completely satisfactory solution since it does not prevent bridging of the gravel.
Other suggestions involve plugging the liner perforations with cement or other material which is later removed by an acid treatment or other separate removal operation. Complicated liner designs involving special liner flanges or valves at every liner joint have also been proposed. In addition, it has been suggested in U.S. Pat. No. 3,999,608 to Smith to provide material such as a semi-solid gel between the wash pipe and the liner in order to temporarily plug the liner openings until the top of the gravel pack reaches the openings. At that point the semi-solid gel offers less resistance to flow than the gravel pack itself, resulting in the liquid breaking through the gel and flowing down to the open end of the wash pipe. These various methods, however, either slow the gravel packing operation or make it less efficient. In the case of the method utilizing semi-solid gel to temporarily plug the liner openings, the gel is purposely designed not to withstand the higher pressures sometimes encountered at locations other than at the perforations adjacent the top of the gravel pack, and so cannot be relied upon under all operating conditions.
It is therefore an object of the invention to cause slurries to have a consistent flow pattern and to prevent bridging of the gravel during a gravel pack operation.
A further object is to accomplish these goals by an economical and efficient process which does not slow the gravel placement process.